Reverse motion detector

ABSTRACT

To detect when an object begins to move in a direction opposite or reverse to its normal direction of movement, a magnetic actuator is disposed on the object and a sensor including first and second reed switches is disclosed adjacent to the object. The reed switches are co-planar in the sensor and disposed such that the first reed switch closes before the second reed switch upon relative movement of the object therepast in its reverse direction. In one embodiment, a logic circuit provides an output signal if, and only if, the second reed switch closes in response to this actuator movement after the first reed switch closes and while the first reed switch remains closed. The output signal initiates operation of a timing circuit which provides a timing signal that inhibits further operation of the logic circuit and is applied through an audio oscillator to drive an amplifier and a loud speaker for a short time period. When the timing signal ceases, the logic circuit is automatically reset to detect subsequent reverse object movements. In a preferred embodiment, the apparatus is used as a vehicle back-up alarm in which the magnetic actuator is attached to the vehicle&#39;&#39;s drive shaft and the sensor is disposed in proximity thereto.

Unite States Patent 91 Brown [451 Apr. 17, 1973 [73] Assignee: ElectroDevelopment Corporation,

Lynwood, Wash.

[22] Filed: Dec. 2, 1970 [21] Appl. No.: 94,424

[52] US. Cl. ..340/70, 340/271, 340/384 E [51] Int. Cl. ..B60q 5/00 [58]Field of Search ..340/70, 223, 271,

[56] References Cited UNITED STATES PATENTS 2,592,968 4/1952 Soule..340/223 UX 2,656,106 10/1953 Stabler ..340/271 UX 2,685,082 7/1954Beman et al.... ..340/271 2,733,431 1/1956 Steele ..340/271 X 2,940,0636/1960 Atkinson ..340/ 70 3,120,655 2/1964 Bcason et al. ..340/2713,304,434 2/1967 Koster ..340/271 X 3,325,782 6/1967 Der ..340/223 UX3,431,553 3/1969 Osmond 340/223 X 3,503,042 3/ 1970 Skinner ..340/673,569,927 3/1971 Guyton et a1 ..340/271 X Primary Eraminer-John CaldwellAssistant Examiner-Kenneth N. Leimer Att0rneyChristensen, Sanbom &Matthews ABSTRACT To detect when an object begins to move in a directionopposite or reverse to its normal direction of movement, a magneticactuator is disposed on the object and a sensor including first andsecond reed switches is disclosed adjacent to the object. The reedswitches are co-planar in the sensor and disposed such that the firstreed switch closes before the second reed switch upon relative movementof the object therepast in its reverse direction. In one embodiment, alogic circuit provides an output signal if, and only if, the second reedswitch closes in response to this actuator movement after the first reedswitch closes and while the first reed switch remains closed. The outputsignal initiates operation of a timing circuit which provides a timingsignal that inhibits further operation of the logic circuit and isapplied through an audio oscillator to drive an amplifier and a loudspeaker for a short time period. When the timing signal ceases, thelogic circuit is automatically reset to detect subsequent reverse objectmovements. in a preferred embodiment, the apparatus is used as a vehicleback-up alarm in which the magnetic actuator is attached to the vehiclesdrive shaft and the sensor is disposed in proximity thereto.

8 Claims, 4 Drawing Figures PATENTED APR 1 1 ms SHEET 1 BF 2 INVENTOR.ROBERT A. BR

* WC/ZLATOA AM TIM/N6 [06/6 ME/JMS' BYMV/ ATTORNEYS PATENTED APR 1 7I973 SHEET E OF 2 M m 0 WR v3 m wfifi m @m E W 0 I. K m5 Q Q 5 V N MQ 'mU NWGW \U M Q, G W@ ir M M w w m g REVERSE MOTION DETECTOR FIELD OF THEINVENTION This invention generally relates to apparatus for detectingthe motion of an object, and, more particularly, to such an apparatusproviding an output signal useful for alarm or control purposes when theobject moves in a direction opposite or reverse to that normallyencountered.

BACKGROUND OF THE INVENTION In many situations in which there isrelative movement of mechanical objects, movement in a directionopposite or reverse to that normally present must be detected. Thedetection may be for purposes providing a control signal to associatedapparatus. For example, it may be desirable to detect reverse motion ofa machine tool carriage in order to initiate movement of the work pieceupon which the machine tool has been acting for further machiningoperations. Or, it may be desirable to detect when the shaft of themachine has begun to rotate in a reverse direction to initiate a controlsignal stopping all rotation thereof.

In other cases of translative or rotary movement, detection of reversemovement is desirable for reasons of safety. Particular examples of thisapplication are devices which provide an audible indication when avehicle is proceeding in a reverse direction. There have been manyinstances of serious injury or death to construction personnel becauseof the lack of an adequate warning when a vehicle is being backed up.Because of these hazardous conditions, many safety codes now requirethat construction vehicles be provided with such warning devices. Somecodes require that an audible warning be given at the initiation of thevehicles movement in a reverse direction, and others require that anaudible indication be given for every predetermined increment of vehiclemovement.

Although reverse direction indicators are known to be prior art, theyhave not proved satisfactory when used as vehicle backup detectors.First, the devices have not been rugged enough to meet external uses ofthis type. Second, the devices have not been sensitive enough to providean alarm or a control indication at the initiation of the reversemovement. For example, those devices generally sense rotary movement ofan object in a reverse direction, but only when the speed of rotationthereof exceeds a predetermined minimum, such as rpm. Third, the devicesare generally not adaptable to existing equipment, but must be customdesigned for each installation. Fourth, the devices are not versatileenough to provide an output indication for control or alarm purposesupon rotary or translative movement of an object in a reverse direction.

It is therefore an object of this invention to provide an apparatuswhich provides an indication whenever an object is moved in a directionopposite or reverse to that normally encountered, whether throughtranslative or rotary movement.

It is further object of this invention to provide an apparatus whichprovides an output indication approximately at the initiation of suchopposite or reverse movement.

It is another object ofthis invention to provide an apparatus whichprovides an output indication for every passage by a sensor of theobject in the opposite or reverse direction.

It is still another object of this invention to provide a vehicleback-up detector which is adaptable to any vehicle having a rotationalmechanism controlling the reverse direction of movement thereof forproviding an audible alarm upon such movement.

It is yet another object of this invention to provide a vehicle back-updetector which provides an audible alarm for every predeterminedincrement of movement of the vehicle in a reverse direction.

It is another object of this invention to provide a vehicle back-updetector whose audible alarm is repeated with a frequency proportionalto the speed of the vehicle in a reverse direction.

It is a further object of this invention to provide a vehicle back-updetector which is simple of application, rugged, and reliable.

SUMMARY OF THE INVENTION These objects and others are achieved, briefly,by providing an apparatus including first and second, coplanartransducers which are disposed to be actuated in response to therelative movement of an actuator, and a logic circuit which provides anoutput signal, if, and

only if, the first and second transducers are actuated thereby in apredetermined sequence.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can perhaps best beunderstood by reference to the following portion of the specification,taken in conjunction with the accompanying drawings in which:

FIG. 1 is a pictorial diagram showing an application of the invention asa construction vehicle back-up detector;

FIG. 2 is a pictorial diagram showing in greater detail the invention ofFIG. 1;

FIG. 3 is a block diagram of an embodiment of an electronic systemforming part of the invention; and

FIG. 4 is a schematic diagram corresponding to the block diagram of FIG.3.

DESCRIPTION OF A PREFERRED EMBODIMENT Now referring to FIG. 1, aconstruction vehicle 10 is illustrated which includes a drive shaft 12coupled to rear driving wheels 16 and 18 by a differential and axlecombination 14. A sensor element 20 is supported from differential 14 bya mounting bracket 22 so as to be physically adjacent an actuator 24which is mounted on drive shaft 12. An electrical cable 26 couples thesensor 20 to a junction box 28 which is affixed to a loud speaker 30. InFIG. I, loud speaker 30 is in turn attached to a portion of the vehiclesframe 32 and disposed at the rear of the vehicle 10.

As is commonly known, the drive shaft 12 is rotated every time the rearwheels 16 and 18 move, in a direction corresponding to the direction ofrotation thereof. lBy sensing the direction of rotation of drive shaft12, a reliable criterion for sensing when the vehicle 10 is moving in areverse direction can be established.

With particular reference now to FIG. 2, the physical elements of thesensor 20 comprises a housing 34 of a non-magnetic, metallic material,such as aluminum, in which are imbedded first and second reed switchesS1 and S2. Preferably, the housing 34 is covered with an epoxy pottingcompound. Reed switches S1 and S2 are preferably of amagnetically-actuated type. The longitudinal dimension of S1 and S2 arearranged in parallel within housing 34 in a plane P1, and separatedtherein by a distance D1.

The actuator 24 comprises a fixture 36, also preferably of anon-magnetic, metallic material, in which are defined first and secondrecesses 38, 39. Straps 40, 42 are passed through recesses 38, 39 andsecure the fixture 36 to drive shaft 12. A third recess 46 is providedin fixture 36 and contains a permanent bar magnet 44 whose north-south(N-S) axis is preferably parallel to the longitudinal dimensions of reedswitches S1 and S2. Fixture 36 and magnet 44 are preferably covered withan epoxy potting compound. When drive shaft 12 has rotated to a pointwhere the actuator 24 is directly under sensor 20, the upper surface ofmagnet 44 is separated from plane P1 by a distance D2.

Cable 26 is connected to the electrical contacts pro vided in S1 and S2and serves to connect, through connector 46, these contacts to theremainder of the detector system which is disposed within junction box28. The system components withinjunction box 28 are also preferablycovered with an epoxy potting compound. Loud speaker 30 may be anycommercially-available, weatherproof transducer and is secured to frame32 by a mounting bracket 48. The construction vehicle is such that thedrive shaft 12 rotates in a counterclockwise direction F when the wheels16 or 18 are rotating so as to move the vehicle 10 in a forwarddirection. Likewise, drive shaft 12 is rotated in a clockwise directionR when the wheels 16 or 18 are rotating such that the vehicle 10 ismoving in a reverse direction.

The physical relation of the components shown in FIG. 2 is such thatreed switch S1 is actuated before reed switch S2 when shaft rotation isin direction R, and reed switch S2 is actuated before reed switch 81when shaft rotation is in direction F. To obtain signals useful indetermining which of the directions F or R is present, certain designconsiderations are important. First, to insure that one reed switch canbe actuated before the other upon shaft rotation, the N-S axis of magnet44 should be aligned with the longitudinal axes of switches S1 and S2,and the separation distance D1 should be carefully chosen. Second, thedistance D2 must be such that both switches S1 and S2 can be actuated bythe passage of the magnet 44, and in addition, at some point, besimultaneously actuated thereby.

With these design considerations, the sequence of closure of switches S1and S2 can then be used to determine whether drive shaft 12 is rotatingin direction F or direction R.

The logical sequence of switch closures must be as follows. First,switch 511 closes before switch S2 closes. Second, switch S2 closeswhile switch S1 is still closed. Third, switch S1 opens before switch S2opens. By providing appropriate logic circuitry to provide an outputsignal if, and only if, this sequence is followed, detection of reversemotion can be accomplished.

Now turning to FIG. 3, a system for providing an audible alarm via loudspeaker 30 in response to rotation of the drive shaft 12 in direction Ris illustrated. Signals obtained from the closures of switches S1 and S2are connected to the inputs of a logic means 50. To

detect the aforementioned sequence of switch closures, logic means 50may incorporate any one of three distinct logic statements. These are:(a) switch S1 closes while switch S2 is open; (b) switch S2 closes whileswitch S1 is closed; switch S1 opens while switch S2 is closed.

Logic means 50 accordingly incorporates circuitry to provide an outputsignal if one of these logic statements is fulfilled. Such an outputsignal is applied to a timing means 52 which in turn produces a timingsignal on a line 53 for a predetermined period of time after theapplication of the output signal from logic means 50 thereto. Thispredetermined period of time may be short enough such that one timingsignal is provided for each output signal from logic means 50. In onecase, the timing period was chosen to equal 0.3 seconds.

The timing signal on line 53 is applied to logic means 50 to inhibit itsfurther operation during the timing period and to an oscillator 54 sothat an output signal having an audible frequency is provided therefromfor the timing period. The output signal from oscillator 54 is amplifiedin an amplifier 56 and applied directly to loud speaker 30 so that anaudible warning is given to personnel in the immediate vicinity of thevehicle 10.

When the production of timing signal on line 53 ceases, the audiblewarning stops due to de-actuation of oscillator 54 and logic means 50 isenabled for further operation. If the vehicle 10 continues to move in areverse direction, the drive shaft 12 will move actuator 24 in aclockwise direction away from sensor 20 such that the aforementionedlogic statement cannot be fulfilled. However, actuator 24 will againapproach the sensor 20 upon the next revolution thereof so as to repeatthe sequence of switch closures and, in doing so, fulfill the logicstatement incorporated in logic means 50 so that an output signal isprovided thereby to initiate the production of an audible warning byloud speaker 30.

lt can be immediately recognized by those skilled in the art that suchan audible warning is provided at every predetermined incrementaldistance of travel of the vehicle 10 in a reverse direction. Moreover,the frequency of audible warnings is directly proportional to the speedof rotation of the drive shaft 12 in the direction R, and thus to thespeed of travel of the vehicle 10 in a reverse direction. In such amanner, construction personnel are apprised not only of the fact thatthe vehicle 1(1) is backing up but also of the relative speed with whichit is moving and of the relative separation between themselves and thevehicle. Most important, the audible warning is provided at almost theinstant that vehicle 10 begins to move in the reverse direction. Thedistance for vehicle 10 to move backwards before warning is given isrelated to the relative positions of actuator 24 and sensor 20 when thedrive shaft motion begins. If an even more immediate alarm is desired, aplurality of sensors 20 could be provided spaced at various intervalsaround drive shaft 12, or a plurality of actuators 24 could be disposedin a similar manner with a single sensor 20.

A working model of the system illustrated in FIG. 3 was constructed anda schematic diagram thereof is illustrated in FIG. 4. The reed switchesS1 and S2 are shown as each having a normally-open pair of contacts, oneside of which are connected together with a common lead, and the otherside of which are connected as separate inputs to the logic circuit 50.The common lead is in turn connected directly to a ground bus. In putsto the ground bus and to a voltage supply bus may be obtained from thevehicles DC battery, with the positive terminal of the battery having apotential V thereon connected to the voltage supply bus and the negativeterminal thereof to the ground bus. A diode CR1 provides reverse voltageprotection to the system, such as would be encountered where the batteryconnections to be reversed or were there to be highly-inductive,negative switching transients on the supply lines. A circuit includingresistor R1 and capacitor C1 is connected to further block noise andtransients on the battery line from affecting the logic means 50 andother elements of the system. The voltage across capacitor C1 is in turnregulated by a zener diode CR2 to a value required for proper andreliable operation of the system.

Switch S1 is coupled to a first input of a NAND gate G1 by an inputcircuit including resistors R3, R4 and capacitor C2, and switch S2 iscoupled to a first input of a second NAND gate G2 by an input circuitincluding resistors R2, R5, and capacitors C3, C4, and resistor R7. Thesecond inputs to the NAND gates G1 and G2 are supplied from the positivevoltage supply V, through a resistor R6. The outputs of gates G1 and G2are in turn connected to the inputs of a third NAND gate G3.

The combination of resistor R4 and capacitor C2 functions as an inputfilter to block radio frequency interference from the first input ofgate G1 and the combination of resistor R2 and capacitor C3 functions asa similar radio frequency interference filter for gate G2. In addition,it is desirable that gate G2 have additional suppression for contactbounce of switch S2 upon opening thereof, and this function is providedby resistor R5 and capacitor C3.

The gates G1 G3, together with their associated input components andconnections, incorporate one of the three aforementioned logicstatements necessary for determination of whether or not the sequence ofswitch closures is correct. The logic statement incorporated is thatlisted as b) above, and in which an output signal is provided whenswitch S2 closes while switch S1 is closed. This statement isimplemented in FIG. 4 by positive logic, but it is obvious to thoseskilled in the art that any type of logic may be used.

In operation, each of the NAND gates G1 G3 provides a logic output, if,and only if, both inputs thereof have a logic I supplied thereto. Forall other cases, the outputs remain at a logic 1 level. In FIG. 4, alogic 0 corresponds to a voltage potential approaching ground potential,and a logic 1 corresponds to a voltage potential approaching that of thesupply voltage V,. Normally, the outputs of gates G1 and G2 are logicOs, because the signal to the second inputs are derived from V throughresistor R6 and the input signals to the first terminals thereof arederived from V, through R3 R4 C2 and R5 C3 C4, respectively. Therefore,the output of gate G3 is normally a logic I. When reed switch S1 isfirst closed upon rotation of drive shaft 12 in direction R, the groundpotential applied therethrough appears at the first input of gate G1 asa logic 0, thereby changing the output state to a logic 1.

However, the output of gate G3 remains at a logic 1 because of the logic0 output from gate G2. The output of gate G1 remains at logic l for theduration of closure of switch S1.

When switch S2 is actuated, the ground pulse applied therethroughappears as a logic 0 at the input to gate G2 and accordingly the outputthereof is also switched to a logic I. If switch S1 is simultaneouslyclosed and the output of gate G1 is a logic I, the output of gate G3 nowshifts to a logic 0. However, the output of gate G2 remains at logic Ionly for the brief instant that is required for capacitor C4 to becomecharged so that DC gate current is thereafter blocked from the firstinput thereof. At such a time, the output of gate G2 reverts to a logic0.

When gate G3 changes to a logic 0 output, logic zeros are providedthereby which are coupled to the second inputs of gates G1 and G2 sothat the outputs of gates G1 and G2 remain latched in a logic 1 stateuntil otherwise reset.

If switch S2 closes first, however, the output of gate G2 changes to alogic I as previously described. However, the time period of chargingfor capacitor C4 is such that the output of gate G2 reverts to a logic 0before switch S1 can close and the output of gate G1 changes to a logicI. In such a case, the output of gate G3 remains at a logic I.

In this manner, detection of the appropriate logic statement isaccomplished such that only the sequence of switch closures whichresults from rotation in direction R provides a logic 0 output from gateG3.

The logic 0 output state from gate G3 functions as the output signalfrom logic means 50 and is supplied to timing circuit 52 which includesa normally conducting transistor Q2 which maintains a transistor O1 in anonconducting condition. A connection is made from resistor R6 directlyto the collector of transistor Q1 and through a resistor R15 to the baseof a transistor Q4 which forms one element of oscillator 54. Once theoutput of gate G3 becomes a logic 0, a charging path is provided for acapacitor C5 through gate G3, a resistor R8, and a resistor R10. Duringthe resultant period of charging thereof, as determined by the timeconstant of the C5 R10 combination, a negative voltage is applied to thebase of transistor Q2 to place that transistor in a non-conductingcondition.

When transistor Q2 becomes non-conducting, bias current is supplied totransistor Q1 by a resistor R9 connected to the voltage supply V,,,thereby placing transistor O1 in a conducting condition. A this time,the timing signal in the form of ground potential appearing at thecollector of O1 is coupled to the second inputs of gates G1 and G2 andto the base of transistor Q4. This action results in the aforementionedlatching of gates G1 and G2 at a logic I output state so that furtheroperations of switches S1 and S2 have no effect whatsoever on the logicmeans 50 and, in addition, turns transistor Q4 on to initiate theproduction of the output signal from oscillator 54. Transistor Q4,together with its associated transistor Q3 and other components, form asquarewave multivibrator which produces a pulsed DC output at thecollector of transistor Q4. In one embodiment, the frequency of thissquarewave oscillation was approximately 1700 Hz.

The component values and connections in oscillator 54 are chosen suchthat the squarewave output at the collector of transistor O4 issymmetrical, despite unsymmetrical loading of the output stage by theamplifier 56 and loud speaker 30.

The embodiment of the amplifier 56 comprises a conventional Darlingtonpair Q5, Q6 which amplify the squarewave output to a level suitable forproducing an audible warning from loud speaker. 30 of sufficientacoustic level. in one embodiment, this acoustic level was approximatelyl db, as measured 5 feet from the centerline of loud speaker 30.

Various modifications and other applications of the reverse motiondetector of this invention will be apparent to those skilled in the art.For example, other logic statements can be used in logic means 50, aslong as detection is accurately and reliably made of the aforementionedsequence of switch closures corresponding to object motion in anopposite or reverse direction. The invention obviously has applicabilityto any situation where opposite or reverse motion is to be detected andused for alarm or control purposes. In such situations, the outputsignal from logic means 50 or the timing signal from timing means 52 maybe used for alarm or control purposes. in other embodiments, the sensorcould be disposed on the object and the actuator be relativelystationary thereto. Also, it may be desirable to use other types ofproximity switches and principles of actuation thereof than the magnetiCally-actuated reed switches shown in the embodiment of FIGS. 1 and 2.Therefore, it should be clearly understood by those skilled in the artthat the invention is not limited to the embodiment heretoforedescribed, but rather is intended to be bounded only by the limits ofthe appended claims.

Iclaim:

l. A detector for providing an indication when a vehicle is proceedingin a reverse direction, comprising:

a sensor, including first and second spaced switches;

a rotational mechanism, the direction of rotation of said mechanismcorresponding to the direction of movement of the vehicle, said sensorbeing stably positioned at a predetermined location relative to saidrotational mechanism; means fixedly attached to said rotationalmechanism for actuating said first and second switches in apredetermined sequence; logic means responsive to said first and secondswitches for providing an output signal if, and only if, said first andsecond switches are actuated in a predetermined sequence, said logicmeans comprising a first gate providing a signal whenever said firstswitch is actuated, a second gate providing a signal for a short periodof time after the actuation of said second switch, and a third gateproviding said output signal only upon coincidence of said signals fromsaid first and said second gates;

timing means responsive to said logic means for producing a timingsignal for a predetermined period of time after the production of saidoutput signal; and

means actuated by said timing means for providing a warning for theduration of said timing signal. 2. A detector as recited in claim 1further comprising means for inhibiting the operation of said logicmeans for the duration of said timing signal.

3. A detector as recited in claim 1 wherein said means for providing awarning comprises an oscillator providing an audible square wave signalin response to said timing signal, a loudspeaker, and an amplifier meansdriving said loudspeaker in response to said audible, square wavesignal.

4. A detector as recited in claim 1, wherein said first and secondswitches both comprise reed switches of a magnetically actuable type,and said actuator includes a permanent magnet.

5. An apparatus for detecting motion of an object in a directionopposite or reverse to that normally encountered comprising:

a sensor including first and second switches spaced apart in apredetermined fashion, said first and second switches being actuated ina predetermined sequence in response to the reverse movement of theobject;

logic means responsive to said first and second switches for providingan output signal if, and only if, said first and second switches areactuated in the predetermined sequence, said logic means comprising afirst gate providing a signal whenever said first switch is actuated, asecond gate providing a signal for a short period of time after theactuation of said second switch, and a third gate providing said outputsignal only upon coincidence of said signals from said first and secondgates;

timing means responsive to said logic means for producing a timingsignal for a predetermined period of time after the production of saidoutput signal; and

means connected to said timing means for providing a warning for theduration of said timing signal.

6. A detector as recited in claim 5 further comprising means forinhibiting the operation of said logic means for the duration of saidtiming signal.

7. A detector as recited in claim 5 wherein said means for providing awarning comprises an oscillator providing an audible square wave signalin response to said timing signal, a loudspeaker, and an amplifier meansdriving said loudspeaker in response to said audible, square wavesignal.

8. A detector as recited in claim 5 wherein said first and secondswitches both comprise reed switches of a magnetically actuable type.

1. A detector for providing an indication when a vehicle is proceedingin a reverse direction, comprising: a sensor, including first and secondspaced switches; a rotational mechanism, the direction of rotation ofsaid mechanism corresponding to the direction of movement of thevehicle, said sensor being stably positioned at a predetermined locationrelative to said rotational mechanism; means fixedly attached to saidrotational mechanism for actuating said first and second switches in apredetermined sequence; logic means responsive to said first and secondswitches for providing an output signal if, and only if, said first andsecond switches are actuated in a predetermined sequence, said logicmeans comprising a first gate providing a signal whenever said firstswitch is actuated, a second gate providing a signal for a short periodof time after the actuation of said second switch, and a third gateproviding said output signal only upon coincidence of said signals fromsaid first and said second gates; timing means responsive to said logicmeans for producing a timing signal for a predetermined period of timeafter the production of said output signal; and means actuated by saidtiming means for providing a warning for the duration of said timingsignal.
 2. A detector as recited in claim 1 further comprising means forinhibiting the operation of said logic means for the duration of saidtiming signal.
 3. A detector as recited in claim 1 wherein said meansfor providing a warning comprises an oscillator providing an audiblesquare wave signal in response to said timing signal, a loudspeaker, andan amplifier means driving said loudspeaker in response to said audible,square wave signal.
 4. A detector as recited in claim 1, wherein saidfirst and second switches both comprise reed switches of a magneticallyactuable type, and said actuator includes a permanent magnet.
 5. Anapparatus for detecting motion of an object in a direction opposite orreverse to that normally encountered comprising: a sensor includingfirst and second switches spaced apart in a predetermined fashion, saidfirst and second switches being actuated in a predetermined sequence inresponse to the reverse movement of the object; logic means responsiveto said first and second switches for providing an output signal if, andonly if, said first and second switches are actuated in thepredetermined sequence, said logic means comprising a first gateproviding a signal whenever said first switch is actuated, a second gateproviding a signal for a short period of time after the actuation ofsaid second switch, and a third gate providing said output signal onlyupon coincidence of said signals from said first and second gates;timing means responsive to said logic means for producing a timingsignal for a predetermined period of time after the production of saidoutput signal; and means connected to said timing means for providing awarning for the duration of said timing signal.
 6. A detector as recitedin claim 5 further comprising means for inhibiting the operation of saidlogic means for the duration of said timing signal.
 7. A detector asrecited in claim 5 wherein said means for providing a warning comprisesan oscillator providing an audible square wave signal in response tosaid timing signal, a loudspeaker, and an amplifier means driving saidloudspeaker in response to said audible, square wave signal.
 8. Adetector as recited in claim 5 wherein said first and second switchesboth comprise reed switches of a magnetically actuable type.